The invention concerns a piggable tube and a method for producing such a tube according to the preamble of the independent claims. More specifically, it deals with a tube, which can be used to deliver electrically conductive coating material by means of a pig into the vicinity of parts of the system that are grounded or set to a low voltage in a system for mass-production coating of workpieces.
As is known, such tubes are needed in coating systems for the electrostatic mass-production coating of workpieces, such as vehicle chassis, with water enamel or other conductive coating material, which can be set to a high voltage on the order of 100 kV. The coating material is moved through the tube by the pig, with the pig also having to act as a perfect separating body between its own liquid or gaseous pushing medium and the delivered coating material (DE 198 30 029, EP 0 904 848, EP 1 172 152, EP 1 108 475, etc.).
Piggable tubes have special requirements, such as, among other things, flexibility, fatigue strength under reverse bending stress, chemical resistance against the media, such as conventional paint and fluids acting as flushing and/or pushing media for the pig, which can be corrosive under some circumstances and which is moved by the pig, as well as low friction values and high resistance to wear and tear relative to the passing pig and the tube itself. Another requirement is a precisely dimensioned inner diameter with high shape and dimensional accuracy also for relatively high pressures. Also important is extremely low adhesion for the transported fluids, which must be removed without residue from the inner wall for setting up insulating sections in the line formed by the tube. The inner wall of the tube should be extremely smooth. Due to these requirements, specially suited plastics must be used, which, however, have the disadvantage that these plastics do not also have optimum electrical insulating properties.
Therefore, in a coating system of the mentioned class, if piggable tubes for media at a high voltage are to be laid close to grounded parts of the system or can accidentally come close to grounded parts, then the problem arises of the risk of voltage breakdown or at least undesirably high leakage currents. Consequently, until now special high-voltage-resistant pipes or tubes have been used for lines subject to this risk in electrostatic coating systems, which are not piggable or are only poorly piggable due to their material, also, among other things, because it is difficult to manufacture them precisely with an inner diameter corresponding to the pig.
Another problem is the risk of damage to piggable tubes from external effects. From DE 100 63 234, it is known to arrange a piggable tube coaxially in an outer pipe with greater internal diameter for protection primarily against kinks and damage blocking the path of the pig, wherein a protective pad is created with compressed air between the two pipes. This kink-protection tube is not suitable for high-voltage isolation.